1. Field of the Invention
The present invention relates to sensing, transmitting and processing of electrical signals carried by an optical carrier wave. More particularly, the present invention relates to modulating optical carriers, having sidebands, with relatively weak signals, and improving the power ratio of the sidebands relative to their carrier wave by the use of a Brillouin medium.
2. Description of the Prior Art
The processing systems that utilize fiber optics and electro-optical modulators to modulate information carried by optical carrier waves and their sidebands are of ever increasing importance. The modulation efficiency of electro-optical modulators has a direct impact on the efficiency of the optical processing systems. One of the contributing factors that degrades the modulation efficiency is that most modulators exhibit large switching (on-off) voltages and so small input signals have little effect on the optical carrier transmitted from the electro-optical modulator. The modulator output commonly consists of a strong unmodulated optical carrier with relatively weak signal sidebands.
The strong unmodulated optical carrier is further typically and undesirably strengthened because it is desired to have high optical power levels in order to create relatively high signal-to-noise ratios of the signals being carried by the optical carrier. However, these strong unmodulated optical signals that correspond to weakly modulated optical waves (having small modulation indices, commonly referred to as a depth of modulation) leave significant unmodulated signal power in the original carrier. Hence, despite a high average optical power needed to increase the electrical signal power for desired signal-to-noise ratios, the electrical power carried by the modulated signals may be quite small. The unmodulated signal, that is, excess optical power not being utilized for modulation purposes is detrimental in at least two ways. First, optical amplifiers are limited by both their average input and output powers; therefore, optical amplifiers will have limited use to boost the weakly modulated optical wave contained in the carrier signal. More particularly, the high unmodulated signal sets the operating value of the amplifier which may not allow for the amplification of the low modulated signal having a need for amplification. Second, and even more important, the average optical power must be kept below approximately 5 milliwatts (mW) to avoid signal reduction, distortion and damage to the photodetector that commonly receives the output of the electro-optical modulator. This 5 mW limitation does not take advantage of the existing optical signal generator having average optical power outputs approaching 100 to 200 mW levels.
Accordingly, it is desired that an apparatus and a method of operation thereof be provided to reduce the unmodulated carrier power so that amplifiers can be efficiently used and so that optical carrier generators having relatively high output power levels may be detected by a photodetector without any operation degradation of the photodetector or without any damage to the photodetector. If such an apparatus and method are provided then optical amplifiers may be more efficiently utilized and a corresponding reduction in the fiber-optical link loss may be realized. Furthermore, the provided apparatus and method may be utilized in conjunction with other optical systems that utilize linear carrier filtering techniques so as to make additional improvements in the fiber-optic link efficiency. If such an apparatus and method of operation thereof are provided the sensitivity and efficiency of the overall fiber-optical link or its related system may be improved.